In a moving body such as a vehicle or an aircraft, for example, an antenna apparatus in which a patch antenna is mounted as one of the antennas for a radar that monitors the surroundings is used. This antenna apparatus includes a dielectric substrate and a patch antenna formed on the substrate. The patch antenna typically has a configuration in which a patch radiating element (patch-shaped conductor) is formed on the dielectric substrate. In addition, a conductor portion is typically formed on a surface of the dielectric substrate on the side opposite the surface on which the patch radiating element is formed. The conductor portion functions as a ground plane. The surface of the dielectric substrate on which the patch radiating element is formed is hereinafter referred to as a substrate front surface. The surface of the dielectric substrate on the side opposite the surface on which the patch radiating element is formed is hereinafter referred to as a substrate back surface. Furthermore, a conductor portion may also be widely formed on the substrate front surface as well, so as to reach a substrate end portion, separately from the patch radiating element.
In a patch antenna configured in such a manner, when the patch antenna is operated, a current (surface current) flows to the ground plane surface as a result of an electric field formed between the patch radiating element and the ground plane. Furthermore, the surface current is transmitted to the substrate end portion and is diffracted at the substrate end portion. In addition, emission (radiation) from the substrate end portion occurs due to the effects of the diffraction wave. When the conductor portion is formed on the substrate front surface, the surface current also flows to the conductor portion and causes emission from the substrate end portion. The emission from the substrate end portion caused by the surface current becomes unnecessary emission that affects performance of the patch antenna. That is, directivity of the patch antenna becomes disturbed as a result of the emission from the end portion.
In regard to the foregoing, a technology for suppressing the surface current flowing to the ground plane is disclosed in PTL 1. Specifically, a plurality of conductive patches are formed in the periphery of the patch radiating element on the substrate front surface of the dielectric substrate. Each conductive patch is connected in a conductive manner to the ground plane on the substrate back surface by a conductive connecting body (hereinafter referred to as a conductive via) having a circular columnar shape. The structure composed of the conductive patch and the conductive via has a band gap (electromagnetic band gap) that inhibits propagation of the surface current on the ground plane at a specific frequency. The structure composed of the conductive patch and the conductive via is hereinafter referred to as an electromagnetic band gap (EBG).
As a result of numerous EBGs being formed in the periphery of the patch radiating element as described above, propagation of surface current to the substrate end portion can be suppressed. Therefore, disturbance in the directivity of the patch antenna can be suppressed